Polycarbonate resins have a wide range of applications as thermoplastic resins with good impact resistance, thermal resistance, flame retardance, and transparence. They are lighter and more readily processed than inorganic glass. Thus, by imparting a light-diffusing property, they can be suitably employed in applications requiring light diffusion, such as lighting covers, illuminated signs, light-passing screens, various displays, and the light-diffusing sheets and the like of liquid-crystal display devices.
Conventionally, the methods of adding inorganic compounds such as glass, silica, and aluminum hydroxide have been proposed as methods of imparting light-diffusing property to polycarbonate resins. However, when these inorganic compounds are added, there are drawbacks in that thermal stability decreases during molding processing, extrusion processing, and the like, and mechanical strength decreases.
For example, Patent Document 1 discloses a light-diffusing polycarbonate resin to which polymethyl methacrylate microparticles with a mass average particle diameter ranging from 0.5 to 100 μm and a difference in refractive index with the polycarbonate resin of equal to or more than 0.01 are added as a light-diffusing polycarbonate resin in which improvement has been made with regard to such drawbacks. However, this polycarbonate resin affords an inadequate light-diffusing property and poor practical utility.
The addition of polyorganosiloxane particles instead of acrylic resin has also been proposed. For example, Patent Documents 2 and 3 propose polycarbonate resins into which polyalkyl silsesquioxane particles are blended.
The blending in of polyalkyl silsesquioxane particles as proposed in Patent Documents 2 and 3 can enhance transmittance and the light-diffusing property, achieving a certain diffusing effect.
However, the specifications that have been required of light-diffusing polycarbonate resin compositions for optical applications have grown ever more stringent in recent years. Further, a variety of specifications are required for individual applications. Thus, the requirements of various fields cannot be fully satisfied by simply adding polyorganosiloxane particles in this manner. Specifically, there is a need for polycarbonate resin compositions with an overall balance of transmittance, diffusion rate, and degree of dispersion.
A polycarbonate resin composition in which polyorganosiloxane particles of specific average particle diameter and of specific particle size distribution are blended into a polycarbonate resin in a specific ratio to obtain a polycarbonate resin composition with good total light transmittance and light-diffusing property has been proposed (Patent Document 4). The polycarbonate resin composition disclosed in Patent Document 4 has good mechanical characteristics such as impact resistance, as well as good total light transmittance and light-diffusing property. However, it has a drawback in the form of poor thermal resistance, as well as inadequate flame retardance.
Patent Document 5 discloses a high-transmittance light-dispersing plate made of polycarbonate resin with a total light transmittance of equal to or greater than 85% and a diffuse transmittance of 10 to 30% in which microparticles 50 μm or less in diameter and having an average particle diameter of 5 to 20 μm are blended. However, this plate affords a low light-diffusing property and is of poor practical utility.
Patent Document 6 discloses a polycarbonate resin composition to which has been added calcium carbonate, titanium oxide, and polyorganohydrogensiloxane as a resin composition with good light transmittance and light-diffusing property. However, this composition is unsatisfactory in terms of thermal stability and impact resistance and does not afford adequate total light transmittance.
Patent Document 7 discloses a directly backlit light-diffusing plate made of polycarbonate resin comprised of a composition into which transparent microparticles with an average particle diameter of 1 to 30 μm and a fluorescent brightening agent have been blended. However, this plate enhances brightness through the use of a fluorescent brightening agent. Thus, thermal stability is inadequate, coloration occurs during molding and extrusion, and discoloration tends to occur during recycling. It is thus of little practical utility.
Patent Document 8 discloses a polycarbonate resin composition affording good total light transmittance, good light-diffusing property, and good moldability without loss of mechanical properties, such as the inherent impact resistance of polycarbonate resins, in the form of a polycarbonate resin composition into which are blended in a specific ratio acrylic resin microparticles of specific average particle diameter and specific particle diameter distribution. However, this composition presents the drawback of poor thermal resistance and inadequate flame retardance.
Patent Documents 9 to 11 and the like disclose flame retardant polycarbonate resin compositions comprised of two types of polycarbonate resin—a high molecular weight and a low molecular weight polycarbonate resin—and a flame retardant. However, the flame retardant polycarbonate resin compositions that are disclosed in these Patent Documents contain polycarbonates with a viscosity average molecular weight of equal to or greater than 100,000 and blended in flame retardant quantities of 0.1 weight part or more. They also present problems in the form of the flame retardance and transparence of molded articles.
[Patent Document 1] Japanese Unexamined Patent Publication (KOKAI) Heisei No. 03-143950
[Patent Document 2] Japanese Patent No. 3,263,795
[Patent Document 3] Japanese Patent No. 4,220,829
[Patent Document 4] Japanese Unexamined Patent Publication (KOKAI) No. 2011-57786
[Patent Document 5] Japanese Unexamined Patent Publication (KOKAI) Heisei No. 05-257002
[Patent Document 6] Japanese Unexamined Patent Publication (KOKAI) 2000-169695
[Patent Document 7] Japanese Unexamined Patent Publication (KOKAI) No. 2004-029091
[Patent Document 8] Japanese Unexamined Patent Publication (KOKAI) No. 2011-157536
[Patent Document 9] Japanese Unexamined Patent Publication (KOKAI) Heisei No. 2-251561
[Patent Document 10] Japanese Unexamined Patent Publication (KOKAI) Heisei No. 9-59505
[Patent Document 11] Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-323118